The present invention relates to a deterioration diagnosis system for an air-fuel ratio sensor to increase or decrease an output linearly in accordance with an air-fuel ratio of an internal combustion engine.
In a deterioration diagnosis system for an air-fuel ratio sensor applied to an internal combustion engine having an air-fuel ratio sensor for sensing an air-fuel ratio of emission gas, the system disclosed in Japanese laid-open patent publication No. 2003-20989 is comprised of: a fuel recovery operation detecting means for detecting that the engine is under a fuel recovery condition after fuel cut operation; a variation calculating means for calculating a variation of the air-fuel ratio in a predetermined variation calculating duration based on an output of the air-fuel ratio sensor every fuel recovery operation; a variation value determining means for comparing a maximum value of a plurality of variations calculated as described above with a predetermined judgment reference value and for determining abnormality when the maximum value is smaller than the predetermined judgment reference value.